The present invention relates to electronic circuits, and more particularly to a differential amplifier with a rail-to-rail input stage and a controlled output common mode voltage.
Differential amplifiers are well known and widely used in electronic circuits to amplify the difference between a pair of differential input signals. FIG. 1 is a transistor schematic diagram of a differential amplifier 10, commonly referred to as the emitter-coupled pair. Current source 20 coupled to the emitter terminals of transistors 16 and 18 is used to bias amplifier 10. As is well known, amplifier 10 has a differential mode gain, defined by the difference of the values of the input signals INP and INN, as well as a common mode gain, defined by the average of the values of the input signals INP and INN. Accordingly, output voltage Vout is determined both by the difference as well as the average of the values of the input signals INP and INN.
A differential amplifier may be used as an input stage of an operational amplifier to provide, among other things, a relatively high differential gain, a relatively high common-mode rejection ratio, and a relatively high common mode range. It would be desirable to have a differential amplifier adapted to receive and amplify a rail-to-rail differential input voltage and that is further characterized by a controlled output common mode voltage.